I already saw some cod1 maps mentioned and I feel the need to mention one of the most impressive cod:uo custom maps I've seen: uo_broadsword, influenced by the movie where eagles dare, featuring a huge alpine landscape with a castle on a mountaintop only reachable by cable cars.
http://callofduty.filefront.com/file/UO_Broadsword;60856
Knowing the limitations of the engine, the sheer size and scale of this map amazes me.